1. Field of the Invention
This invention relates to a process of thermally treating fine-grained solids, particularly of preheating, calcining and burning ground raw material for making cement, wherein the solids are preheated in a multistage heat exchanger by means of a mixture of exhaust gases from a furnace and a reaction vessel, which mixture is conducted in two parallel exhaust gas lines, and the solids are conducted between heat exchange stages associated with one of said exhaust gas lines and heat exchange stages associated with the other exhaust gas line in alternation, the solids are subsequently heated further only by the exhaust gas stream from the furnace, and are subsequently supplied to the reaction vessel and thereafter to the furnace, and the exhaust gas from the furnace which has been used to heat the solids in the heat exchange stage which directly precedes the reaction vessel in the flow path for the solids is subsequently mixed with the exhaust gas from the reaction vessel to form the mixture of exhaust gas to be conducted through the two exhaust gas lines. The invention relates also to apparatus for carrying out the process.
2. Description of the Prior Art
In the production of cement, the exhaust gas from the furnace should not be supplied to the calcinator and the higher exhaust gas temperature of the furnace should be utilized to preheat the raw ground material in the heat exchanger stage which directly precedes the calcinator in the solids flow path. But an arrangement in which the exhaust gas line associated with one set of heat exchange stages is flown through only by exhaust gas from the calcinator whereas the exhaust gas line associated with the other set of heat exchange stages is supplied only with the exhaust gas from the furnace would not be desirable. It is known from U.S. Pat. No. 4,431,454 that these objects can be accomplished in that the exhaust gas stream from the furnace is caused to deliver heat to the ground raw material and is then mixed with the exhaust gas stream from the calcinator in the heat exchanger stage which directly precedes the calcinator in the flow path for the ground raw material, and the stream of mixed exhaust gases is then evenly distributed to the two exhaust gas lines. The exhaust gases from the furnace and from the calcinator are mixed in a mixing chamber, which necessarily increases the overall height. Moreover, the equal distribution of the stream of mixed gases to the two exhaust gas lines extending through the heat exchanger prevents an adaptation to different conditions in the heat exchange stages associated with the two exhaust gas lines. The raw ground material is transferred in alternation between heat exchange stages associated with one exhaust gas line and heat exchange stages associated with the other exhaust gas line and the ground raw material is subsequently supplied to the calcinator so that the ground raw material is at different temperatures in the heat exchange stages associated with the two exhaust gas lines and different conditions occur in the heat exchange stages associated with the two exhaust gas lines. For an optimization of the process it would be desirable to adapt the flow rates of the exhaust gases in the two exhaust gas lines to the prevailing conditions.